1. Field of Invention
The present invention relates to a bipolar junction device structure and its method of manufacture. More particularly, the present invention relates to a high-voltage compatible, bipolar junction device structure and its method of manufacture.
2. Description of Related Art
In some high-voltage circuit design, parasitic dipolar devices are often required. However, because earlier versions of the vertical bipolar junction transistor (BJT) have a large base width or lack a double diffusion drain (DDD) structure , gain of the transistor is too small. Yet, a lateral bipolar transistor has an even smaller gain, and hence its applications are even more restrictive.
FIG. 1 is a cross-sectional view showing a conventional vertical bipolar junction transistor. In FIG. 1, the substrate 100 is an N-doped region. The substrate 100 serves as a collector for the bipolar transistor, and the N.sup.+ region 102 acts as a contact area for the collector. The well 104 is a P-doped region acting as a base, and the P.sup.+ regions 106 serve as contact areas for the base. The emitter 108 is another N.sup.+ region within the P-well 104.
The structure shown in FIG. 1 is a typical npn vertical bipolar junction transistor. When proper voltages are applied, electrons are emitted from the emitter 108 through the base 104 and inject into the substrate 100, which serves as a collector. The path taken by the electrons is known as the base width and is represented by w in FIG. 1. Since base width of a conventional vertical BJT is rather large, gain of the transistor is small. Hence, efficiency of the device is low.
FIG. 2 is a cross-sectional view showing a conventional lateral bipolar junction transistor. In FIG. 2, the substrate 200 is an N-doped region. The substrate 200 has a well region 202. The well 202 is a P-doped region serving as a base. The P.sup.+ region 204 within the well 202 serves as a contact area for the base. The emitter 206 and the collector 208 are two N.sup.+ regions formed on each side of the P.sup.+ base region 204 within the well 202. The structure shown in FIG. 2 is a typical npn lateral bipolar junction transistor.
Due to the presence of a contact area 204 between the emitter 206 and the collector 208, the rate of movement of electrons from the emitter 206 to the collector 208 is greatly reduced. Moreover, some electrons may be absorbed within the contact area 204. Therefore, gain of the lateral bipolar junction transistor is even less than the vertical BJT and more restrictions are imposed upon circuit applications.
In light of the foregoing, there is a need to provide an improved bipolar junction transistor structure.